Haiku Alpha 3 has been in development for more than 14 months. In that time more than 800 bugs have been identified and fixed, major sections have been updated, applications have been added and updated, and great progress has been made in supporting additional hardware. Here is a summary of updates, more details can be found here. Also inside, interviews with some core Haiku developers.

I haven't followed Haiku over the past couple years due to lack of mindspace, but when the project started (openbeos.org!), Haiku was using the work of a former BeOS engineer: a fairly straightfoward fork of the NewOS kernel -- http://newos.org/features.php

How much that of that code/architecture has been retained someone else probably is better equipped to answer.